The separation of subpopulations of biological cells by partition in phase-separated aqueous polymer solutions is proving to be a uniquely sensitive and versatile technique. We are proposing to systematically study the mechanisms which determine cell partition in dextran/poly(ethylene glycol) systems. Since cell surface charge has been implicated as one determinant of partition we will investigate the Donnan potential which appears between phases containing certain ions. The potential between the phase boundary and each of the bulk phases will be estimated from electrophoretic mobility measurements in dilute phase emulsions. Interfacial tensions will be determined by the rotating drop technique. Phase polymer adsorption to standard cells will be measured as will zeta potentials of cells suspended in isolated phases. This information will be used to test a thermodynamically derived expression for the partition coefficient. The counter current distribution technique used for cell separations will be optimized and cell partition chromatography using the phase systems will be investigated. Finally, because partition is apparently sensitive to age-related membrane properties, optimized phase systems will be utilized to obtain fractions of human erythrocytes of differing in vivo ages in order to study their membrane properties. This work constitutes part of an ongoing investigation into the mechanism of removal of senescent red cells from the circulation.